The present invention relates generally to devices for transporting a flexible article, and, more particularly, to devices for transporting a thin laminate through one or more processing stations.
Flexible articles often need to be held generally flat for various processing operations. An exemplary flexible article is a thin laminate which is processed to be part of a printed circuit board. Referring to FIG. 1, a plurality of thin laminates 10A-D are shown. These laminates 10 are stacked and coupled together to form a printed circuit board 12. In some circuit boards 12 between 10 and 20 laminates 10 are stacked together to form the circuit board 12. As is known, each of laminates 10 may be processed to provide electrical connections and components on the laminate 10. Further, the electrical connections for a given laminate may be connected to the electrical connections of other laminates 10 in the final printed circuit board 12.
Printed Circuit Board (PCB) fabrication involves a sequence of process steps that are performed on the thin laminates 10. The type and number of steps depends on the complexity of the PCB being fabricated, however, most if not all multilayer PCB boards will require a photolithography operation and an surface treatment operation.
Photolithography involves coating a copper-clad laminate with a thin photosensitive film called photoresist. After coating the laminate, it is placed in an exposure unit, where areas of the photoresist can be exposed with light to polymerize that area. Other areas of the laminate which do not receive the light will remain unpolymerized. The next step, commonly known as DES (Develop/Etch/Strip), is where unexposed photoresist will be washed away (Develop), the copper will be etched (Etch), and the remaining exposed photoresist will be washed away (Strip). This leaves a circuit pattern of copper left on the laminate which provides electrical connections for the PCB.
The developer portion of DES will subject the photoresist covered laminate to a caustic solution, typically potassium carbonate. Any unpolymerized photoresist is broken down and rinsed away leaving a polymerized photoresist pattern on the laminate which protects the copper underneath. Upon exiting the developer processing machine, the laminate may be inserted into a copper etching machine wherein exposed copper is etched by a Cupric Chloride or Ammoniacal solution. The copper which is protected by the polymerized photoresist remains unharmed. Upon exiting the etcher processing machine, the laminate is rinsed and transported into a photoresist stripping machine which exposes the laminate to an aggressive base solution. In the photoresist stripping machine, the photoresist's molecular bonds produced during the exposure step are torn apart and rinsed away leaving the copper circuit image on the laminate.
Often times a single machine performs the DES processes. One such machine is a horizontal conveyorized spray line in which laminates are supported by rollers which pass the laminate from roller to roller as they rotate. A laminate which is thick spans across the rollers, even while getting sprayed by solution from sprayers located above and below the rollers. However, thin laminates often cannot span from one roller to the next because they are too flimsy. This results in the front edge of a thin laminate commonly falling between the rollers. Rollers that are closer together will transport a thin laminate, but the presence of the rollers themselves will block the sprayer reducing the likelihood of adequate solution coverage. This results in the circuit image being incorrect. In one example, s-wheels may be added to the conveyor to reduce the amount of blockage of the thin laminate relative to the spray.
PCB process engineers often advise operators to use adhesive tape to fasten a thin laminate to an item commonly referred to as a leader board. The leader board will pull or tow the thin laminate through a conveyor. The leader board does not get caught in the rollers because it is thicker and more rigid. An exemplary leader board is a thicker dielectric. A problem with using the leader board is that the tape must be removed from the thin laminate at the end of the conveyor. This takes time and care because the operator must ensure that the thin laminate is not distorted when the tape is removed. In another example, the thin laminate is taped to a frame member which is thicker than the thin laminate. The thin laminate is typically taped at each of the of four corners. Once again, the tape must be removed at the end of the processing which takes operator time and may distort the thin laminate.
Another step in fabricating printed circuit boards after the circuit pattern is etched on a laminate is surface treatment (commonly called oxide coating). The practice of surface treatment promotes better adhesion between laminates when pressed together to form a PCB. The surface treatment is typically accomplished with a vertical dip tank, but may be accomplished with horizontal conveyor lines, as well. A single dip tank typically holds about 50 gallons of solution. Processes that require a series of solutions will have a line of dip tanks, each tank containing a solution for cleaning, coating, or rinsing the part. An automated hoist system may be employed to move a laminate from tank to tank and to leave the laminate in a given tank a preset amount of time.
The hoist system includes a basket into which multiple laminates are loaded. The laminates must be kept separated during the dipping process. This is not a problem with thick laminates. A problem for thin laminates is that during dipping laminates do not remain flat. Warping of the thin laminate will occur, and the mechanical agitation of the solution will cause waviness for the thin laminate. To counter this, the thin laminate may be taped to a frame made of a thicker dielectric. However, this once again causes additional operator time and potential damage to the thin laminate when the tape is removed. Further, if multiple thin laminates are nearby in the same basket, the thin laminates will tend to stick together by natural fluid adhesion. This disrupts fluid from passing along the surface of the thin laminate and therefore causes the coating of thin laminate to be uneven or inconsistent.